Semiconductor device manufacturing processes normally include a step in which a metal film is formed at the surface of the processing target material such as a semiconductor wafer (hereafter may be simply referred to as a “wafer”). A metal film must be formed when, for instance, forming a wiring pattern at a wafer surface or filling recesses (via holes) between wirings or recesses (contact holes) for substrate contact. The metal film may be a thin film formed by depositing metal or a metal compound such as W (tungsten), WSi (tungsten silicide), WN (tungsten nitride), Ti (titanium), TiN (titanium nitride) or TiSi (titanium silicide).
The resistance of the metal film used for wiring purposes or the like should be as low as possible. The tungsten film among the metal films listed above, which has a particularly low specific resistance and becomes set at low temperature for film formation, is deemed desirable from this viewpoint and, accordingly, it is widely used when filling recesses between wirings and substrate contact recesses.
The tungsten film is usually formed through deposition by using WF6 (tungsten hexafluoride) as a metal base material gas and reducing the metal base material gas with a reducing gas such as hydrogen, silane or difluorosilane. In addition, before forming the tungsten film, a thin, uniform barrier layer to act as a base film, constituted with a TiN film deposited on the wafer surface or a laminated film (TiN/Ti film) constituted with a Ti film and a TiN film deposited on the Ti film, is formed to assure better adhesion of the tungsten film and deter a reaction with the lower wiring metal layer or the substrate. The tungsten film is then deposited over the barrier layer.
In order to assure good filling of recesses or the like with a tungsten film, a hydrogen gas with a lower reducing property than silane is normally used as the reducing gas. The use of a hydrogen gas as the reducing gas in the tungsten film formation may result in the formation of a volcano or a void in a hole (e.g., a contact hole).
FIG. 10 illustrates the mechanism of volcano formation. FIGS. 10A, 10B and 10C show how the reaction occurs in sequence. FIG. 10 indicates that the behavior of fluorine is a significant factor in volcano formation and that TiF3 produced through the reaction with fluorine at the barrier layer moves upward and erupts through the upper layer. More specifically, the fluorine component in the pre-reaction WF6 gas reacts with the barrier layer, and through this reaction, a titanium fluoride, primarily TiF3, is produced (see FIGS. 10A and 10B). This titanium fluoride expands, erupting upward through the barrier layer as a volcano (see FIG. 10C).
In order to prevent such volcano formation, an initial tungsten film to act as a nulcleation layer is formed before depositing a main tungsten film so as to protect the base barrier layer from the attack by the WF6 gas during the formation of the main tungsten film. The pre-reaction WF6 gas must be removed promptly and also, the fluorine concentration in the tungsten film must be controlled so that the fluorine component in the tungsten film does not directly react with the base barrier layer during the formation of the initial tungsten film.
The initial tungsten film may be formed through an atomic layer deposition (ALD) method whereby the WF6 gas and a reducing gas constituted with B2H6 (diborane) gas are alternately supplied multiple times with a purge step executed between the WF6 gas supply step and the B2H6 gas supply step (see, for instance, patent reference literature 1). The resistance of the tungsten film deposited through this method is low, with low fluorine concentration in the tungsten film and, accordingly, it is assumed that the formation of a fluorine compound through a reaction with the base metal can be prevented.
Gas supply modes adopted in the related art to supply the individual types of gases during the tungsten film formation are now explained in reference to drawings. FIG. 11A presents an example of a gas supply mode adopted in the related art, and FIG. 11B presents another example of a gas supply mode adopted in the related art. FIG. 11A shows a gas supply mode adopted in the ALD method mentioned earlier, in which a B2H6 gas is used as the reducing gas. FIG. 11B shows a gas supply mode with a SiH4 gas used as the reducing gas. FIG. 12 illustrates steps through which a hole is filled with a tungsten film formed by adopting either of the gas supply modes shown in FIG. 11.
The following explanation is provided by assuming that the B2H6 gas is used as the reducing gas (see FIG. 11A). It is to be noted that an Ar gas and an N2 gas to be used as a carrier gas and a purge gas respectively are supplied at constant flow rates and that the processing pressure is sustained at a constant level through the entire processing phase. In addition, a barrier layer 4 is formed over the entire surface of a wafer M, which includes the inner surface of a hole 2, e.g., a contact hole, as shown in FIG. 12A.
First, an initial tungsten film 8 is formed at the wafer M shown in FIG. 12A by alternately supplying the B2H6 gas and the WF6 gas repeatedly over brief periods. During this process, a purge step is executed to eliminate any residual gas in the container between a B2H6 gas supply step and the subsequent WF6 gas supply step. The WF6 gas molecule layer absorbed on the wafer surface during the WF6 gas supply step is reduced with the B2H6 supplied through the subsequent step and a tungsten film constituted with several atomic layers is formed through a single pair of gas supply steps. By repeating the alternating gas supply steps a given number of times, the initial tungsten film 8 with a desired film thickness is formed, as shown in FIG. 12B.
Next, the main tungsten film formation step is executed by simultaneously supplying the WF6 gas and an H2 gas, thereby filling the hole 2 with a main tungsten film 10 deposited as shown in FIG. 12C. In the alternative gas supply mode shown in FIG. 11B, SiH4 (mono-silane) is used in place of B2H6 (see, for instance, patent reference literature 2). In the alternative gas supply mode, the initial SiH4 gas supply step may be executed over an extended period of time relative to the subsequent other SiH4 gas supply steps and, in such a case, during the initial SiH4 gas supply step, initiation processing for depositing a decomposition intermediate such as SiHx (0≦x≦4) at the wafer surface is also executed.    (Patent reference literature 1) Japanese Laid Open Patent Publication No. 2002-038271    (Patent reference literature 2) Japanese Laid Open Patent Publication No. 2003-193233